High-quality theoretical potential energy surface for Be2 by using the multireference averaged quadratic coupled-cluster (MR-AQCC) method and large basis sets

Both electron correlation and basis set effects are important in the description of the bond in Be2. We apply a new version of the multireference averaged quadratic coupled-cluster method which is an approximately size extensive modification of the multireference configuration interaction method and was shown to yield potential energy surfaces parallel to full-CI ones. We show this in the case of Be2 as well. Since the method is considerably cheaper than full-CI, we were able to account for core correlation and calculate the whole potential energy surface. Equilibrium geometry, vibrational frequencies and dissociation energy are obtained. For the former two quantities the experimental data are confirmed by the calculations, while the results suggest that the dissociation energy might be considerably higher than the one obtained from the lowest vibrational frequencies. Special attention is paid to the basis set superposition error. Our best calculation of the dissociation energy De is 864 cm−1 which is considered to be a lower bound.